What is HIV?

HIV stands for ‘Human Immunodeficiency Virus’. HIV is a retrovirus that infects cells of the human immune system (mainly CD4 positive T cells and macrophages–key components of the cellular immune system), and destroys or impairs their function. Infection with this virus results in the progressive depletion of the immune system, leading to ‘Immune Deficiency’.

The immune system is considered deficient when it can no longer fulfill its role of fighting off infection and diseases. Immunodeficient people are much more vulnerable to a wide range of infections, most of which are very rare among people without immune deficiency. Diseases associated with severe immunodeficiency are known as ‘Opportunistic Infections’, because they take advantage of a weakened immune system.

Origin of HIV
The AIDS epidemic was discovered June 5 1981, when the U.S. Centers for Disease Control and Prevention reported a cluster of pneumonia in five homosexual men in Los Angeles.

Three of the earliest known instances of HIV infection are as follows:

A plasma sample taken in 1959 from an adult male living in what is now the Democratic Republic of Congo.

HIV found in tissue samples from a 15 year old African–American teenager who died in St. Louis in 1969.

HIV found in tissue samples from a Norwegian sailor who died around 1976.

Two species of HIV infect humans: HIV–1 and HIV–2. HIV–1 is more virulent and more easily transmitted. HIV–1 is the source of the majority of HIV infections throughout the world, while HIV–2 is not as easily transmitted and is largely confined to West Africa.

HIV–a lentivirus
HIV is a lentivirus, and like all viruses of this type, it attacks the immune system. Lentiviruses are in turn part of a larger group of viruses known as retroviruses. The name ‘Lentivirus’ literally means ‘Slow Virus’ because they take such a long time to produce any adverse effects in the body. They have been found in a number of different animals, including cats, sheep, horses and cattle. However, the most interesting lentivirus in terms of the investigation into the origins of HIV is the Simian Immunodeficiency Virus (SIV) that affects monkeys.

It is now generally accepted that HIV is a descendant of a Simian Immunodeficiency Virus because certain strains of SIV’s bear a very close resemblance to HIV–1 and HIV–2, the two types of HIV.

HIV–2 for example corresponds to SIVsm, a strain of the Simian Immunodeficiency Virus found in the sooty mangabey (also known as the green monkey), which is indigenous to western Africa.

The more virulent, pandemic strain of HIV, namely HIV–1, was until recently more difficult to place. Until 1999, the closest counterpart that had been identified was SIVcpz, the SIV found in chimpanzees. However, this virus still had certain significant differences from HIV.

About HIV–2
Until recently, the origins of the HIV–2 virus had remained relatively unexplored. HIV–2 is thought to come from the SIV in Sooty Mangabeys rather than chimpanzees, but the crossover to humans is believed to have happened in a similar way (i.e. through the butchering and consumption of monkey meat). It is far rarer, significantly less infectious and progresses more slowly to AIDS than HIV–1. As a result, it infects far fewer people, and is mainly confined to a few countries in West Africa.

Although a variety of theories exist explaining the transfer of HIV to humans, there is no widely accepted scientific consensus of any single hypothesis and the topic remains controversial.